With continuous reduction in the size of a semiconductor device, enhancing the carrier mobility of channel becomes a very important technique. In the design of a stress layer for a substrate, different materials have different characteristics such as lattice constant, dielectric constant, forbidden gap, particularly carrier mobility, etc., as shown in Table 1 below.
TABLE 1LatticeDielectricForbiddenMobility (cm2/V-s)Materialconstant (nm)constantgap(eV)electronholeSi0.543111.81.121600430Ge0.5675160.6639001900GaAs0.565312.41.429200400InAs0.605814.80.3640000500InSb0.64817.70.1777000850
It can be seen from Table 1 that among the above possible materials for substrate, Ge has the highest hole mobility and a relatively higher electron mobility. Using Ge as the substrate, particularly the channel region of a semiconductor device will greatly enhance the carrier mobility, thus enabling manufacture of a higher-speed large scale integrated circuit (LSIC).
Further, it can also be seen from Table 1 that Ge has a similar lattice constant as that of the material Si, thus Ge can be easily integrated on a Si substrate commonly used in the semiconductor technology, such that a semiconductor device with better performance can be manufactured by the technology without making great improvements thereto, thereby improving the performance while reducing the cost at the same time.
However, in the prior art, the MOSFETs with the channel region formed of Ge or other non-Si materials all comprise a large area of high-mobility materials deposited in the active region on the Si substrate or completely take high-mobility materials as the substrate, that is, the high-mobility materials can be used in not only the channel region but also the source/drain areas. Actually, it is enough to improve the device response speed by increasing the carrier mobility in the channel region only, and the process cost will be unnecessarily increased if the source/drain areas also adopt non-Si materials. Besides, high-mobility materials such as Ge has a resistivity higher than that of Si, causing the source-drain parasitic series resistance to be increased, thus suppressing the improvement of the device performance to some degree, but the traditional process of taking the metal silicide as source-drain contacts is also hard to be applied to the sources/drain formed by the non-Si high-mobility materials.
Overall, the existing semiconductor device with Si channel has poor performance and reliability, the carrier mobility in the channel region shall be further improved to improve the electrical performance and reliability of the semiconductor device, while the process shall be saved and the cost shall be reduced.